Non-HT PPDU Structure

IEEE^® 802.11™¹ is a packet-based protocol. Each packet, also called a physical layer protocol data unit (PPDU), contains preamble and data fields. The preamble fields contain the transmission vector format information. The data field contains the user payload and higher layer headers, such as medium access control (MAC) fields and cyclic redundancy check (CRC). The transmission vector format and the PPDU structure vary between 802.11 versions. The transmission vector (TXVECTOR) format parameter is classified as:

non-HT to specify a PHY implementation that predates the 802.11n™ standard. Non-HT means non-high-throughput.
- Section 17 of [1] defines and describes the OFDM PHY layer and PPDU for non-HT transmission. In addition to supporting non-HT synchronization, the non-HT preamble fields are used in support of HT, VHT, HE, and EHT synchronization.

The table shows 802.11 versions that WLAN Toolbox™ supports, along with the supported TXVECTOR options and associated modulation formats.

802.11 Version	Transmission Vector Format	Modulation Format	Bandwidth/MHz
802.11b™	non-HT	DSSS/CCK	11
802.11a™	non-HT	OFDM only	5, 10, 20
802.11j™	non-HT	OFDM only	10
802.11p™	non-HT	OFDM only	5, 10
802.11g™	non-HT	OFDM	20
802.11g™	non-HT	DSSS/CCK	11
802.11n (Wi-Fi 4)	HT_MF, Non-HT	OFDM only	20, 40
802.11ac™ (Wi-Fi 5)	VHT, HT_MF, Non-HT	OFDM only	20, 40, 80, 160
802.11ah™	S1G	OFDM only	1, 2, 4, 8, 16
802.11ad™	DMG	Single Carrier and OFDM	2640
802.11ax™ (Wi-Fi 6)	HE	OFDMA	20, 40, 80, 160
802.11ba™	WUR	MC-OOK	20, 40, 80
802.11be™ (Wi-Fi 7)	EHT	OFDMA	20, 40, 80, 160, 320

To create non-HT PPDUs and waveforms, use the wlanNonHTConfig object.

A non-HT PPDU is made up of preamble and data portions. The legacy preamble fields (L-STF, L-LTF, and L-SIG) are common to non-HT, HT, VHT, HE, and EHT preambles.

The structure of a non-HT format PPDU

PPDU Field Abbreviation	Description
L-STF	Non-HT Short Training field
L-LTF	Non-HT Long Training field
L-SIG	Non-HT SIGNAL field
Data	The non-HT Data fields includes the service bits, PSDU, tail bits, and pad bits

Non-HT (Legacy) Preamble Fields

The legacy short training field (L-STF) is the first field of the 802.11 OFDM PLCP legacy preamble. The L-STF is a component of EHT, HE, VHT, HT, and non-HT PPDUs.

L-STF location within legacy preamble.

The L-STF duration varies with channel bandwidth.

Channel Bandwidth (MHz)	Subcarrier Frequency Spacing, Δ_F (kHz)	Fast Fourier Transform (FFT) Period (T_FFT = 1 / Δ_F)	L-STF Duration (T_SHORT = 10 × T_FFT / 4)
20, 40, 80, 160, and 320	312.5	3.2 μs	8 μs
10	156.25	6.4 μs	16 μs
5	78.125	12.8 μs	32 μs

Because the sequence has good correlation properties, receivers use it for start-of-packet detection, coarse frequency correction, and setting the AGC. The sequence uses 12 of the 52 subcarriers that are available per 20 MHz channel bandwidth segment. For 5 MHz, 10 MHz, and 20 MHz bandwidths, the number of channel bandwidth segments is one.

The L-LTF is the second field in the 802.11 OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, VHT, HT, and non-HT PPDUs.

The L-LTF, second in the legacy preamble

Channel estimation, fine frequency offset estimation, and fine symbol timing offset estimation rely on the L-LTF.

The L-LTF is composed of a cyclic prefix (CP) followed by two identical long training symbols (C1 and C2). The CP consists of the second half of the long training symbol.

The cyclic prefix followed by the two long training symbols in the L-LTF

The L-LTF duration varies with channel bandwidth.

Channel Bandwidth (MHz)	Subcarrier Frequency Spacing Δ_F (kHz)	Fast Fourier Transform (FFT) Period (T_FFT = 1 / Δ_F)	Cyclic Prefix or Training Symbol Guard Interval (GI2) Duration (T_GI2 = T_FFT / 2)	L-LTF Duration (T_LONG = T_GI2 + 2 × T_FFT)
20, 40, 80, 160, and 320	312.5	3.2 μs	1.6 μs	8 μs
10	156.25	6.4 μs	3.2 μs	16 μs
5	78.125	12.8 μs	6.4 μs	32 μs

The L-SIG is the third field of the 802.11 OFDM PLCP legacy preamble. This field is a component of EHT, HE, VHT, HT, and non-HT PPDUs. It consists of 24 bits that contain rate, length, and parity information. The L-SIG field uses BPSK modulation with rate 1/2 binary convolutional coding (BCC).

The L-SIG in the legacy preamble

The L-SIG consists of OFDM symbol with a duration that varies with channel bandwidth.

Channel Bandwidth (MHz)	Subcarrier Frequency Spacing, Δ_F (kHz)	Fast Fourier Transform (FFT) Period (T_FFT = 1 / Δ_F)	Guard Interval (GI) Duration (T_GI = T_FFT / 4)	L-SIG Duration (T_SIGNAL = T_GI + T_FFT)
20, 40, 80, and 160	312.5	3.2 μs	0.8 μs	4 μs
10	156.25	6.4 μs	1.6 μs	8 μs
5	78.125	12.8 μs	3.2 μs	16 μs

The L-SIG contains packet information for the received configuration.

Packet information in the L-SIG

Bits 0 through 3 specify the data rate (modulation and coding rate) for the non-HT format.

Rate (Bits 0–3)	Modulation	Coding Rate (R)	Data Rate (Mb/s)
Rate (Bits 0–3)	Modulation	Coding Rate (R)	20 MHz Channel Bandwidth	10 MHz Channel Bandwidth	5 MHz Channel Bandwidth
1101	BPSK	1/2	6	3	1.5
1111	BPSK	3/4	9	4.5	2.25
0101	QPSK	1/2	12	6	3
0111	QPSK	3/4	18	9	4.5
1001	16-QAM	1/2	24	12	6
1011	16-QAM	3/4	36	18	9
0001	64-QAM	2/3	48	24	12
0011	64-QAM	3/4	54	27	13.5

For HT and VHT formats, the L-SIG rate bits are set to '1 1 0 1'. Data rate information for HT and VHT formats is signaled in format-specific signaling fields.

Bit 4 is reserved for future use.
Bits 5 through 16:
- For non-HT formats, specify the data length (amount of data transmitted in octets) as described in Table 17-1 and Section 10.27.4 IEEE Std 802.11-2020.
- For HT-mixed formats, specify the transmission time as described in Sections 19.3.9.3.5 and 10.27.4 of IEEE Std 802.11-2020.
- For VHT formats, specify the transmission time as described in Section 21.3.8.2.4 of IEEE Std 802.11-2020.
Bit 17 has the even parity of bits 0 through 16.
Bits 18 through 23 contain all zeros for the signal tail bits.

Note

Signaling fields added for HT (wlanHTSIG) and VHT (wlanVHTSIGA, wlanVHTSIGB) formats provide data rate and configuration information for those formats.

For the HT-mixed format, Section 19.3.9.4.3 of IEEE Std 802.11-2020 describes HT-SIG bit settings.
For the VHT format, Sections 21.3.8.3.3 and 21.3.8.3.6 of IEEE Std 802.11-2020 describe bit settings for the VHT-SIG-A and VHT-SIG-B fields, respectively.

Non-HT Data Field

The non-high throughput Data (non-HT Data) field is used to transmit MAC frames and is composed of a service field, a PSDU, tail bits, and pad bits.

The Non-HT Data field

Service field — Contains 16 zeros to initialize the data scrambler.
PSDU — Variable-length field containing the PLCP service data unit (PSDU).
Tail — Tail bits required to terminate a convolutional code. The field uses six zeros for the single encoding stream.
Pad Bits — Variable-length field required to ensure that the non-HT data field contains an integer number of symbols.

Processing of an 802.11a data field is defined in Section 17.3.5 of [1].

References

[1] IEEE Std 802.11-2020 (Revision of IEEE Std 802.11-2016). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” IEEE Standard for Information Technology — Telecommunications and Information Exchange between Systems — Local and Metropolitan Area Networks — Specific Requirements.